This invention relates to the manufacture of sulfuric acid by a process which comprises catalytic oxidation of wet sulfur dioxide containing gases, and more particularly to an improved process in which the heat of absorption of the components of the wet conversion gas is recovered in useful form.
Manufacturing of sulfuric acid is predominantly carried out by the dry gas contact process, in which sulfur is typically burned in dry air to produce a stream containing sulfur dioxide, the resultant dry sulfur dioxide is catalytically oxidized to sulfur trioxide, and the sulfur trioxide is absorbed in sulfuric acid. Wet sulfur dioxide-containing gas from other sources, such as ore roasting or spent acid combustion, is normally dried before catalytic conversion of the sulfur dioxide to sulfur trioxide. Combustion air for the sulfur burners is dried by contact with sulfuric acid. In such processes, the heat of condensation of the water vapor in the air is wasted. Moreover, installation and operation of the drying tower requires an allocation of space, a substantial investment, and significant operating expense. Also, the operation of the drying tower complicates the overall process water balance and, under some conditions, requires cross-flow transfers of acid to control that balance.
The manufacture of sulfuric acid by processing wet sulfur dioxide and/or sulfur trioxide containing gas is also known in the prior art. The scheme is sometimes referred to as the wet catalytic process. Wet process plants typically operate on feed stocks such as hydrogen sulfide which produce water and SO.sub.2 when burned. In a typical wet gas process, cooled combustion gases are taken directly to the converter, thus eliminating the extensive capital cost associated with gas purification and drying. However, such plants are not without drawbacks. A very large quantity of acid mist may be formed in the absorption step, making emission control difficult and expensive. Wet gas processes often produce sulfuric acid concentrations of less than 93%, though this can be controlled, for example, by burning a combination of sulfur and hydrogen sulfide.
British patent 471,653 describes a process for producing sulfuric acid from wet sulfur dioxide feed gas. In accordance with the process of the British patent, wet SO.sub.2 gas is filtered and oxidized in a vanadium catalyzed contact unit to produce a wet SO.sub.3 gas at a temperature of 400.degree.-430.degree. C. This gas is contacted in a first liquid/gas contact tower with concentrated sulfuric acid at 98% concentration and a temperature of about 200.degree. C., and then in a second tower with 98% sulfuric acid at a temperature of 60.degree.-70.degree. C. The molecular ratio of sulfur trioxide to water is controlled in the range of 1:1, which produces an acid having a composition in the range of 98% constituting an azeotrope of minimum vapor pressure. The patent suggests that the absorption of water predominates in the first tower, while the absorption of sulfur trioxide occurs predominantly in the second tower. Thus, the reference process does not appear to facilitate recovery of absorption energy at high temperature and, indeed, there is no disclosure or suggestion in the reference that the absorption energy is recovered at all.
Schoubye U.S. pat. no. 4,348,373 describes a process for the preparation of sulfuric acid by condensation of a wet sulfur trioxide gas stream. The stream is fed to a multi-stage tower, in the middle stage of which SO.sub.3 is absorbed in acid circulated from a sump below the absorber stage to a spray head above. Acid overflowing the sump passes through a lower concentrator section through which the inlet SO.sub.3 laden gas is passed, and heat from the inlet gas effects the stripping of water from the acid passing through the concentrator stage, thereby increasing the H.sub.2 SO.sub.4 concentration in the acid which is discharged from the bottom of the tower. Thus, the absorption energy is not recovered for any separate process use, but is instead used for stripping the sulfuric acid stream leaving the tower. Even so, the acid produced in the concentrator stage has a strength only in the range of 93-96%, and acid circulated through the absorber stage has a concentration of 85-88%.
Dorr et al. U.S. pat. no. 4,368,183 describes a wet gas process in which a reaction gas containing sulfur trioxide and water vapor, at an H.sub.2 O/SO.sub.3 mole ratio below one, is first condensed by co-current contact with sulfuric acid in a venturi, the sulfuric acid having a concentration of 98.0-100% and a temperature of at least 95.degree. C. as fed to the inlet of the venturi. Gas leaves the venturi at a temperature of at least 120.degree. C. and is passed through a succeeding countercurrent absorption stage and contacted with trickling sulfuric acid having a concentration of 98-100% and a temperature of 70.degree.-120.degree. C. However, as in the case of British '653, and Schoubye '373, Dorr et al contains no disclosure of recovery of the heat of absorption in useful form.
McAlister and Ziebold U.S. pat. nos. 4,576,813 and 4,670,242 describe processes for absorbing sulfur trioxide in sulfuric acid at high temperature and recovering the absorption heat in useful form by transferring heat from the absorption acid to another fluid, thereby heating the other fluid to 120.degree. C. or higher.
There has been a need in the art for a process which provides the capital savings of the wet gas process while controlling acid emissions and achieving maximum energy efficiency by recovery of the heat of absorption of components of wet conversion gas in sulfuric acid.